Invasive weeds that are ripe for action this time of year

All over King County invasive weeds are on the move, growing in parks, gardens, roadsides, and many other places. Now is a perfect time to get outside, enjoy the sun, and eradicate them!

Below are some important invasive and noxious weeds that are very conspicuous this month. These are generally non-regulated noxious weeds so there isn’t a requirement to control them in King County. However, they are all highly invasive and can cause big problems if left unchecked. With a little sweat, you can help make our neighborhoods healthier, safer, and more vibrant. Check out our website for information on ways to control each of these and other invasive weeds.

1. In full bloom, setting seed soon

The following weeds are now in full flower and starting to go to seed. Controlling them now will prevent them from spreading more.

Poison-hemlock (Conium maculatum) is an 8-10-foot-tall biennial that grows throughout King County, often in disturbed areas with wet soil and sun. It’s also highly toxic to people, livestock, and wildlife, so it’s a good idea to know what this plant looks like. You can identify it by:

  • Hairless, hollow stems with red to purple spots or streaks
  • Fern-like green leaves that give off a distinct musty smell
  • Multiple umbrella-shaped clusters of small, white, 5-petaled flowers at stem ends

For more information on poison-hemlock, see our recent feature about it as the April 2018 Weed of the Month.

Poison-hemlock is full-size and blooming. Look for numerous umbrella-shaped clusters of tiny white flowers, fern-like leaves, and hairless stems with reddish-purple splotches. Photo by Maria Winkler.
Poison-hemlock has reddish-purple splotches on its hairless, hollow stems.
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Young poison-hemlock plants resemble wild carrots, with white roots. All parts of the plant are poisonous.
Poison-hemlock’s bright green, fern-like, musty leaves.

Scotch broom (Cytisus scoparius) is a highly invasive shrub with evergreen stems that grows 6-10 feet tall, frequently in dense stands, and is widespread in King County. It’s been in bloom for a while now, and some plants are starting to produce seeds. You can identify it by:

  • Small, simple or 3-part leaves that often fall off in summer
  • Erect, angled branches with distinct ridges
  • Bright yellow, pea-like flowers all along stems
  • Black, hard seedpods with hairy edges
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Scotch broom is currently in flower and starting to go to seed.
Scotch broom’s simple to 3-part leaves.
Scotch broom forms dense stands throughout King County.
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Two insects, including this seed weevil, Exapion fuscirostre, serve as biocontrol agents for Scotch broom. Photo by Jennifer Andreas / WSU Extension.

Multiflora rose (Rosa multiflora) is a multi-stemmed perennial shrub or vine that reaches 5-10 feet tall. It escapes plantings and grows along creeks and roadsides and can climb high into trees as well as grow as a thicket similar to blackberry. You can identify it by:

  • Upright, arching, bright green to reddish stems, usually with thorns
  • Pinnately compound leaves divided into 5-11 sharply toothed, elliptical leaflets
  • Fringed stipules (small leaf-like growths) in pairs at base of leaf stalks
  • Large, white to pink, fragrant flowers with notched petals
Multiflora rose is in bloom, with some plants producing seeds. Photo by Dan Sorensen.
Multiflora rose’s fringed stipules at base of leaf stalks. Photo by Dan Sorensen.
Multiflora rose’s large, white to pink, fragrant flowers with notched petals. Photo by Dan Sorensen.
Multiflora rose’s pinnately compound leaves divided into 5-11 sharply toothed, elliptical leaflets. Photo by Chris Evans, River to River CWMA.

2. Budding or starting to flower

At a bit earlier stage in their life cycles, these plants are in full bloom.

Himalayan blackberry (Rubus armeniacus), one of our most common noxious weeds, invades disturbed areas, riparian zones, tree farms, open spaces, and a variety of other habitats throughout King County. You can identify it by:

  • Thorny, stout, arching canes, star-shaped in cross-section, that grow up to 15 feet tall and 40 feet long; root at their tips and nodes
  • Large, rounded to oblong, toothed leaves, in groups of 5 when mature (sometimes fewer on young shoots)
  • White to pink flowers that look like those of wild rose
  • Large, edible black fruit
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Blackberry is growing robustly right now, but won’t produce its characteristic juicy fruit until mid- to late summer. Plan your blackberry control so that it has the least impact on animals who benefit from blackberry’s flowers or fruits.
Himalayan blackberry produces large, edible black fruit.
Himalayan blackberry’s white to pink flowers resemble those of wild rose.
Himalayan blackberry has large, rounded to oblong, toothed leaves, in groups of 5 when mature (sometimes fewer on young shoots).

Canada thistle (Cirsium arvense) is a 2-5-foot-tall, branched perennial that spreads mostly via creeping rhizomes, as well as by seed. It grows most often in cultivated or disturbed open areas throughout King County. You can identify it by:

  • Narrow, lobed, crinkled, 2-7-inch-long leaves with spines at edges
  • Relatively small flower heads with pink to lavender flowers in clusters at branch ends
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King County’s Canada thistles starting to bloom. “Canada-Thistle” by Homer Edward Price / CC BY 2.0.
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Canada thistle’s flower heads, with pink to lavender flowers, are smaller than other thistles and grow clustered at branch ends.
Canada thistle’s leaves are narrow, lobed, and crinkled, with spines along their edges.
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Canada thistle grows 2-6 feet tall.

Bull thistle (Cirsium vulgare) is a 2-6-foot-tall, branched biennial found especially in cultivated fields and pastures, as well as along trails and roads. You can identify it by:

  • Deeply lobed, rough, hairy leaves with long, sharp spines at edges and midribs
  • Spiny “wings” on stem at leaf base
  • Gumdrop-shaped flower head surrounded by spines, with pink to magenta flowers, growing singly at stem ends

For more information on these two thistles and their look-alikes—native and invasive—see our recent May 2018 Weed of the Month blog post.

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Like Canada thistle, Bull thistle is bolting and will bloom soon.
Spiny “wings” grow on the stem at leaf bases. Photo by Jennifer Andreas / WSU Extension.
Bull thistle’s leaves are hairy, rough, and deeply lobed, with sharp spines at their edges and midribs.

Field bindweed (Convolvulus arvensis) is a deep-rooted perennial vine that invades farms, fields, turf, and residential areas. It grows along the ground and covers objects it comes across. You can identify it by:

  • Slender, twining stems that reach 6 feet long
  • Smooth, arrowhead-shaped leaves
  • Light pink to white, trumpet-shaped flowers
  • Two small leaf bracts about one inch below flowers
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Field bindweed is growing well and just around the beginning of its bloom. You can identify it by its smooth, somewhat arrowhead-shaped leaves and slender, twining stems. Photo courtesy of Oregon State University.
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Field bindweed has two small leaf bracts about one inch below flowers.
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Field bindweed has light pink to white, trumped-shaped flowers. Photo by OSU.
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Field bindweed has somewhat arrowhead-shaped leaves.

Hedge bindweed (Calystegia sepium), a.k.a. “morning glory,” looks a lot like field bindweed but is more common in urban yards and open spaces. You can distinguish it from field bindweed by:

  • Larger, hairless, more arrow-shaped leaves
  • Larger flowers
  • Shallower roots (although still super deep and extensive!)
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Like field bindweed, hedge bindweed is right around that start of its bloom time. Its leaves are larger and more arrow-shaped than those of field bindweed. Photo courtesy of Oregon State University.
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Like field bindweed, hedge bindweed sprawls over the ground and any objects in its path. Photo by OSU.
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Hedge bindweed’s flower (right) is larger than that of field bindweed (left). Photo by OSU.
Hedge bindweed’s trumpet-shaped flower, larger than that of field bindweed.

3. Not yet flowering, but just about fully grown

This final group of plants is mostly putting energy into growing taller right now. They won’t flower until later in the summer. Timing is key for controlling this group of plants so check out the resources below for more information before getting started.

The four species of invasive knotweeds (Polygonum bohemicum, P. cuspidatum, P. polystachyum, P. sachalinense) are herbaceous perennials that thrive in wet soils, such as riverbanks, but will also grow in dry areas. In King County, they are generally not regulated* but control is highly recommended! They spread primarily via rhizomes, roots, and plant fragments, as well as infrequently by seed.  Learn how to control them by taking a class or watching our videos. You can identify them by:

  • 4-13-foot-tall, green to reddish, hollow, bamboo-like stems that grow in large, dense thickets and die back every year
  • Small, white to green flowers in plume-like clusters
  • Large, woody roots
Invasive knotweeds are rapidly growing as they prepare to flower later in the summer.
Knotweed has 4-13-foot-tall, bamboo-like, green to reddish, hollow stems.
Knotweed primarily spreads via rhizomes, plant fragments, and its large, woody roots.
Knotweed’s white to green flowers grow in plume-like clusters.

For more information about how to identify and control these noxious weeds, follow the links above or visit our website. Feel free to contact us with any questions by calling 206-477-WEED (206-477-9333) or emailing

* Control of Bohemian, Japanese, giant and Himalayan knotweed is only required on the Green River and its tributaries upstream of the Auburn City Limits and on the Cedar River and its tributaries upstream of the Renton City Limits (tributaries included are those defined as Type S, F or N aquatic areas in KCC 21A.24.355). Control of these invasive knotweed species is required up to the ordinary high water mark (or up to the top of the bank if the ordinary high water mark cannot be identified) and in the adjacent buffer area as specified in KCC 21A.24.358. This requirement to control knotweed is contingent upon the noxious weed program or program partners providing knotweed control services in the selected area for affected private landowners who request assistance.